In recent years, projectors provided with a liquid crystal panel, DMD (Digital Micro Miller Device) or the like have appeared. They are being used in such presentation as various announcement meetings, business or the like, and further, are adopted also as the instrument for home theaters. And Japanese Unexamined Patent Application Publication No. 2004-69966 describes a projector, as the projector of this kind, in which the illuminating light amount from a light source (lamp) is changed in accordance with the brightness in a projection room or in which the cross sectional area of the light path is changed by a diaphragm device arranged in the vicinity of the light source lamp to adjust the illuminating light amount without changing a light amount itself emitted from the light source lamp, in order to temporarily weaken during no projection.
Further, Japanese Unexamined Patent Application Publication No. 2006-343512 describes a diaphragm apparatus for projectors in which the light path of the light from the light source can be intercepted during no projection, and further describes that it becomes possible to prevent the liquid crystal from deteriorating if the illuminating light is even temporarily weakened or intercepted during no projection by the diaphragm apparatus when such diaphragm is adopted in the liquid crystal projector.
Further, Japanese Unexamined Patent Application Publication No. 2004-69966 and No. 2006-343512 also describe that it becomes possible to obtain a high contrast image on the screen if such a diaphragm apparatus is provided in the projector and the amount of the illuminating light is decreased in the scene of a dark projected image and is increased in the scene of a bright projected image.
Furthermore, Japanese Unexamined Patent Application Publication No. Hei 8-227102 describes that in case of an over-head projector in which the projecting optical system is formed as a varifocal optical system, it becomes possible to preferably obtain a projected image at high magnification if a diaphragm apparatus is arranged between a stage for putting a copy paper thereon and a light source. Further, Japanese Unexamined Patent Application Publication No. 2005-157162 describes a diaphragm apparatus in which a light intercepting blade (which is also named as light intercepting plate, diaphragm plate, or the like) is adapted to be moved on a plane perpendicular to the light path, like the diaphragm apparatus described in Japanese Unexamined Patent Application Publication No. Hei 8-227102. However, in case of this diaphragm apparatus, the respective light intercepting blade is adapted to be formed by each two divided blades to make the space for housing two light intercepting blades small when the light path is fully opened.
Besides, the diaphragm apparatus described in Japanese Unexamined Patent Application Publication No. 2004-69966, Japanese Unexamined Patent Application Publication No. 2006-343512, Japanese Unexamined Patent Application Publication No. hei-8-227102 and Japanese Unexamined Patent Application Publication No. 2005-157162 are adapted to change the area of light path by reciprocating two light intercepting blades in the opposing directions at the same time. Therefore, the center of the light path results in existing always within the light path if the light path is not completely intercepted. However, it is also known that there is a case in which one light intercepting blade is adapted to move from one lateral position to the other lateral position of the light path over a center thereof, in order to form the smallest light path or to intercept completely the light path at the other lateral position.